The present invention relates to an electrostatic chuck for holding a substrate in a chamber.
Electrostatic chucks, which use electrostatic attraction forces to hold a substrate, have several advantages over mechanical and vacuum chucks. For example, electrostatic chucks reduce stress-induced cracks caused by mechanical clamps, allow processing of a larger portion of the substrate, and can be used in processes conducted at low pressures. A typical electrostatic chuck comprises an electrode covered by a dielectric. When the electrode is electrically charged, an opposing electrostatic charge accumulates in the substrate and the resultant electrostatic force holds the substrate onto the electrostatic chuck. Once the substrate is firmly held on the chuck, a plasma of gas is used to process the substrate.
Certain newly developed plasma processes for the fabrication of integrated circuits are often performed at high temperatures and in highly erosive gases. For example, processes for etching copper or platinum are conducted at temperatures of from 250 to 600.degree. C., compared to temperatures of 100 to 200.degree. C. for etching aluminum. The high temperatures and erosive gases thermally degrade the materials used to fabricate the chucks. The high temperature requirement is met by ceramic materials, such as aluminum oxide (Al.sub.2 O.sub.3) or aluminum nitride (AlN). However, it is difficult to attach the ceramic chuck to chamber components which are typically made from metal because the difference in thermal expansion coefficients of the ceramic and metal can result in thermal and mechanical stresses that can cause the ceramic to fracture or chip. It is desirable to have a system for fastening a ceramic chuck to a chamber without causing excessive thermal stresses between the chuck and the chamber.
In addition, the newly developed processes often require the substrate on the electrostatic chuck to be heated to temperatures higher than those provided by the heat load of the plasma. The high temperatures can be obtained by using a heater, for example, the substrate can be heated by infrared radiation from heat lamps provided outside the chamber. However, it is difficult to pass infrared radiation through the aluminum oxide or metal walls of the chamber. In another approach, as described in U.S. Pat. No. 5,280,156, the electrostatic chuck comprises a ceramic dielectric having both the electrode and the heater embedded therein. However, operating the embedded heater at high power levels can cause the ceramic dielectric covering the electrode to microcrack from the thermal stresses generated by differences in thermal expansion between the ceramic, electrode, and heater. Thus, there is a need for an electrostatic chuck that can be heated to high temperatures without damaging the chuck.
In certain processes, it is also desirable to rapidly cool the substrate in order to maintain the substrate in a narrow range of temperatures, especially for etching interconnect lines that have very small dimensions and are positioned close together. However, temperature fluctuations occur in high power plasmas due to variations in the coupling of RF energy and plasma ion densities across the substrate. These temperature fluctuations can cause rapid increases or decreases in the temperature of the substrate. Also, variations in heat transfer rates between the substrate and chuck can arise from the non-uniform thermal impedances of the interfaces between the substrate, chuck, and chamber. Thus, it is desirable to have an electrostatic chuck that can rapidly cool the substrate to more closely control the temperature of the substrate.
Another problem that frequently arises with conventional electrostatic chucks is the difficulty in forming a secure electrical connection between the electrode of the electrostatic chuck and an electrical connector that conducts a voltage to the electrode from a terminal in the chamber. Conventional electrical connectors have spring biased contacts which can oxidize and form poor electrical connections to the electrode. Moreover, electrical connections formed by soldering or brazing the electrical connector to the electrode often result in weak joints that can break from thermal or mechanical stresses. Thus, it is desirable to have an electrostatic chuck with a secure and reliable electrical connection between the electrode and electrical connector.
Yet another problem frequently arises from the vacuum seal between the electrostatic chuck and the surface of the chamber, especially for high temperature processes. Typically, fluid, gas, and electrical lines extend to the electrostatic chuck through vacuum sealed feedthroughs in the chamber. In conventional chambers, the feedthroughs are vacuum sealed by polymer O-rings that are positioned in grooves extending around their circumference. However, the polymer O-rings often lose their compliance and resilience at high temperatures making it difficult to maintain the integrity of the vacuum seal.
Accordingly, there is a need for an electrostatic chuck that can be operated at high temperatures without excessive thermal or mechanical degradation. There is also a need for an electrostatic chuck that can heat the substrate to higher temperatures than those provided by the heat load of the plasma. There is also a need for an electrostatic chuck having a uniform and low thermal impedance to transfer heat to and from the substrate to allow rapidly heating or cooling of the substrate. There is a further need for an electrostatic chuck having a secure and reliable connection between its electrode and electrical connector. There is also a need for degradation resistant vacuum seal between the electrostatic chuck and chamber.